Original subtitles

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The first topic I want to talk about today is the concept of loops,

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things that have to happen over and over and over again.

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The first type of loop I want to introduce you to is something called the For

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loop. And it can be used really easily in combination with something we learned

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in yesterday's lessons, which is lists. By using a

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for loop like this

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we can go through each item in a list and perform some action with each

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individual item.

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So let's try it out in practice. In order to follow along

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go ahead and fork the day 5 start rebel.

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And once I've explained the concept, then I want you to try it out yourself.

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But first, let's say we had a list called fruits and it contained three items,

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apple, peach, and pear.

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If we wanted to access each item in this list individually and print it out one

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by one, then we would use a for loop.

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So if we start out with the keyword for,

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and then we give a name to a single item.

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So in this case, we might call it fruit.

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And then we use the in keyword

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and finally the list that we want to loop through,

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which is our fruits here.

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Now we cap it off with a colon and we go on to the next line and notice how it's

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indented there.

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Now I'm going to go ahead and print the value of each of the fruit variables.

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And if I run this code,

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you'll see that it loops through my list of fruits and for each of the fruits

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inside the list, it prints it out into the console,

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apple, peach, pear, in that order. Now,

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the important thing to realize here is that basically you can imagine that

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behind the scenes, what this code is doing is

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it's taking this list of fruits

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and it's a signing a variable name, fruit, to each of them.

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So the first time this runs fruit is equal to apple,

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the second time this runs fruit is equal to peach,

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and we can see this even more clearly

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if you run it through the Thonny IDE,

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which I told you about at the beginning of the course. Now,

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if I go ahead and click on the debug icon and I step into each of the steps,

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you can see first it establishes that we have a list of three strings,

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apple, peach, pear, and then we go into the for loop.

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Now notice on the right here I've got all the variables being accounted for.

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And the first one it's noticed is the variable fruits,

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which holds a list of strings. Now,

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as I continue into the for loop,

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notice how it's going to look through this list of fruits

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and it's going to assign the variable name fruit to each of the items starting

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from the first one apple.

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So now by the time it's reached line 3,

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we've already got this variable called fruit

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that's been assigned to the value of apple. And so at this point,

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printing out the value of this fruit

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it's obviously going to print out apple and you'll see in the next step.

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There we go. Now, once I'm done here,

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then it's going to loop back to the start of the for loop

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and now this variable fruit is going to be assigned to the next value inside the

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list of fruits.

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So now notice how the variable fruit is attached to the value peach,

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and then it continues this and so on and so forth until it prints out each one

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of the fruits.

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And this really emphasizes the most important aspect of loops.

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The loop allows us to execute the same line of code multiple times.

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In this case, we're executing the print statement three times,

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but our for loop isn't limited to just executing a single statement.

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We don't just have to print out the name of an item in the list.

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We can execute a whole block of statements multiple times,

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and we can do many things inside this for loop and by inside, I mean indented.

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So let's say that in addition to printing out the fruit,

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I'm going to write another print statement and I'm going to not only print out

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the name of the fruit,

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but I'm also going to say fruit + + "Pie."

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So when I run this code, what do you think will happen?

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Well, it prints out apple, and then it prints out Apple Pie

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And then it goes back to the start and it assigns the variable fruit to the next

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item, peach peach pie, pear pear pie.

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So this is how we can implement a simple for loop that loops through a list

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and assigns a variable name to each of the items in the list in order,

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and then inside the for loop after the colon, after some indentation,

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we can do something with that temporary variable for each of the items.

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Now I've been talking a lot about the concept of being inside a for loop versus

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being outside a for loop. And this is really, really important.

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Whenever you see a colon say in our if statement that we saw previously,

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or the for loop that we're using here,

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the indentation is really,

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really important because if it's indented,

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then it means that it's inside the for loop and these instructions

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all get carried out for as many times as the for-loop will need to repeat.

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Now, if I decided that I wanted to say print my,

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uh, fruits,

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so my list of fruits up here, and I put it inside my for loop,

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so indented after the for loop, then as you can imagine,

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it's going to print that for as many times as the loop runs,

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which is going to be three because there's three items in our list. Now,

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if I had indented that back to the beginning,

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so now it's no longer inside the for loop,

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then it's only going to print once and it's going to print it after the for

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loop is done. So notice how it's doing the whole Apple, Apple pie,

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et cetera. And then once the loop is finished,

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then it jumps to the next line and carries out this instruction.

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So the indentation is really, really important,

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and you have to be really careful with this.

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Have a play around with this code

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and if you're still struggling to see how it works

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then move the code over to Thonny and step through each step one at a time and

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see how it works.

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But I hope you're starting to see how loops are really handy at executing an

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instruction over and over and over again,

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getting the computer to save us time and energy.

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A really good example of when loops would come in really handy is,

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you know, how Bart Simpson gets punished

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and it has to write out a sentence over and over again on the Blackboard. Well,

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if only Bart was a programmer, then he would be able to use loops to do this

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and he would be able to chill in the corner. Now,

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once you're happy with this type of for-loop,

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then head over to the next lesson where I've got a coding exercise

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that's going to put your knowledge to the test. All right.

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So I'll see you there.